Spunbonded fibrous sheets made of multiple plexifilamentary strands of oriented polyethylene film fibrils are disclosed in U.S. Pat. No. 3,169,899 (Steuber). Such sheets are produced commercially by E. I. du Pont de Nemours and Company under the trademark "TYVEK" spunbonded olefin. These sheets have proven useful in diverse applications which take advantage of the sheet's unusually good combination of strength, tear resistance and permeability properties. Often, polyethylene pulps are prepared by cutting up precursor sheets (i.e., unbonded plexifilamentary sheets) into small pieces and beating the cut pieces in an aqueous pulp refiner. Examples of prior art methods for producing such pulps include:
Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 19, 3rd edition, John Wiley & Sons, pp. 420-435 (1982) which describes synthetic pulps as generally being very fine, highly branched, discontinuous, water-dispersible fibers made of plastics. Methods are described for producing synthetic pulps by solution flash-spinning, emulsion flash-spinning, melt-extrusion/fibrillation and shear precipitation. The pulps may be blended with other fibers in an attempt to make papers, sheets or boards by conventional wet-lay papermaking techniques. Such pulps are also identified as being used as bonding agents for certain nonwoven materials such as dry-laid, Rando-Webber formed sheets and wet-laid, Fourdrinier-formed sheets.
U.S. Pat. No. 4,608,089 (Gale et al.) which discloses forming oriented polyethylene film-fibril pulps by cutting flash-spun polyethylene plexifilamentary strands into pieces, forming an aqueous slurry with the pieces and then refining the pieces with disc refiners to form a pulp that is particularly well suited for cement reinforcement. The pulp is prepared from flash-spun plexifilaments which are cut into small pieces and beaten in an aqueous medium. Although these pulps have found some utility in reinforcing cement composites, they are not useful in making high grade polyethylene paper.
U.S. Pat. No. 5,000,824 (Gale et al.) discloses forming improved oriented polyethylene film-fibril pulps for reinforcing various articles. The pulps are prepared from flash-spun, oriented, linear polyethylene, plexifilamentary strands that are converted into small fibrous pieces that are then reduced in length by refining in an aqueous medium to form a fibrous pulp slurry. The pulp slurry is then further refined until an average fiber length of no greater than 1.2 mm is achieved and no more than 25% of the fibrous pulp is retained on a 14-mesh screen and at least 50% of the pulp passes through the 14-mesh screen but is retained by a 100-mesh screen. Various articles are disclosed which can be made from the improved pulp. These include, speciality synthetic papers, reinforced gaskets, reinforced cements, reinforced resinous articles and heat-bonded sheets. Although these pulps have found some utility in reinforcing applications and in producing crude paper hand sheets, they are not useful in making high grade, low basis weight polyethylene paper on conventional continuous wet-lay paper-making equipment.
Some of the problems encountered when trying to make high grade polyethylene paper on conventional continuous paper-making equipment with these types of polyethylene pulps include (1) the pulp tends to stick to the drying surfaces while the paper is being dried and (2) the dried paper tends to tear when handled due to low dry strength caused by inadequate heat fusing. Moreover, during the drying process the sheet may elongate in the machine direction and hang in between the drying surfaces. These problems cause the resulting paper sheet to have low dry strength and poor uniformity (e.g., holes and blotchiness). Although there are some methods available which allow synthetic paper to be made from polyethylene pulp on conventional paper-making equipment, they require unique fibers and process steps. One such example is disclosed in U.S. Pat. No. 4,783,507 (Tokunaga et al.), where the inventive feature rests in the use of two polyethylene pulps, one that melts at 95 C. or below and one that melts at higher temperatures. Paper can be prepared from the two polyethylene pulps on a conventional paper-making machine using drying cans which are heated by 100 C. steam. The polyethylene pulps used to make such paper are prepared by the process of U.S. Pat. No. 3,920,508 (Yonemori) wherein flash-spinning takes place using an emulsion of polyethylene in a solvent of polyvinyl alcohol and water.
In an attempt to minimize sticking and elongation difficulties, a particular method has been disclosed in U.S. Pat. No. 5,047,121 (Kochar). Kochar teaches a process for making high grade polyethylene paper containing at least 97 wt. % polyethylene fibrids on continuous wet-lay papermaking equipment. A pulp furnish of oriented polyethylene fibrids and polyvinyl alcohol fibers are deposited on a forming screen to make a waterleaf sheet. The sheet is dried on drying cans using a very particular drying profile to help reduce sticking and elongation. The sheet is thereafter thermally bonded to provide a polyethylene paper having generally high strength, low defects and good uniformity.
Although the teachings of Kochar have been successful for making high grade polyethylene paper, there are still several processing and quality problems associated with its use. Experience has shown that unless the drying profile is carefully controlled and the drying cans are routinely cleaned, sticking, tearing and stretching can still be significant problems. Also, if a non-permanent release agent is used on the drying cans (e.g., PTFE particles in an oil dispersion), holes will occur in the resulting sheet if the oil-based release agent drips on the sheet as it passes along the drying cans. Because of the nature of the pulp material making up the sheet, 1/8 to 1/2 inch (0.3 to 1.3 cm) holes often appear in the resulting sheet following thermal bonding. These holes typically occur during bonding due to fiber shrinkage caused by agglomerates, pills and/or dirt particles that may be present in the wet-laid sheet. Typically, polyethylene pulps with greater than about 2% defects (i.e., agglomerates or pills manifesting themselves as entanglements of pulp fiber) greatly contribute to holes. Moreover, if there is not enough heat to cause the pulp to fuse together or the pulp was too short, the dry strength of the polyethylene paper is significantly compromised. These problems are especially undesirable in end-use applications (e.g., vacuum cleaner bags) where strength, uniformity and porosity must be carefully controlled.
Clearly, what is needed is a process for producing high grade polyethylene paper from polyethylene pulp on conventional continuous wet-lay paper-making equipment wherein the process and the paper produced thereby do not have the deficiencies inherent in the prior art. The paper should have increased dimensional stability, high strength and superior uniformity (i.e., a very low number of defects such as holes, pills or agglomerates) so that it can be successfully used in critical end-use applications such as microfiltration. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the drawings and the detailed description of the invention which hereinafter follows.